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Mozilla is denying that a bug that crashes Firefox 3.5 is a security vulnerability, countering earlier reports that the company's latest browser contained a flaw even though it had just been patched.

In a Sunday post to Mozilla's security blog, Mike Shaver, the company's vice president of engineering, said that the bug, which had originally been disclosed on the milw0rm hacker site, is not a vulnerability. "The reports by press and various security agencies have incorrectly indicated that this is an exploitable bug," Shaver said. "Our analysis indicates that it is not, and we have seen no example of exploitability."

The bug, continued Shaver, does crash Firefox 3.5 -- and the recently-released 3.5.1 -- in some situations. But there's no way for an attacker to exploit that by injecting malicious code on the machine. The bug can crash Windows, Mac and Linux editions of Firefox, including Firefox when it's being run on the still-unfinished Windows 7.

Both Shaver in his blog post and developers on Bugzilla noted that the Firefox crash on Macs was due to a flaw in Apple's operating system, specifically the ATSUI system library. "We have reported this issue to Apple, but in the event that they do not provide a fix we will look to implement mitigations in Mozilla code," Shaver said.

Mozilla developer Vladimir Vukicevic countered that it was unlikely Apple would fix the problem. "We've reported this and similar bugs in the past to Apple; they have so far had no interest in fixing such bugs in their font rendering subsystems, especially if they're in ATSUI and not CoreText," said Vukicevic on Bugzilla.

Another Mozilla hand suggested that the Mac OS X bug may affect other browsers as well. "Chances are more applications use the same buggy API (Safari? Chrome?)," Andreas Gal said. Gal, a project scientist at the University of California-Irvine, was a key contributor to the TraceMonkey JavaScript engine that Mozilla added to Firefox with Version 3.5.

Just last Thursday, Mozilla patched Firefox 3.5 for the first time, issuing a fix for a critical vulnerability in TraceMonkey's just-in-time (JIT) compiler. In the run-up to creating a fix for that flaw, Mozilla developers speculated that the hacker had dug through Bugzilla to find information that helped him exploit the vulnerability.

Mozilla repeated the charge in the entry for the newest bug. "Sam and Reed think that someone might be trawling Bugzilla in order to develop exploits," said Mike Beltzner, the director of Firefox, in a comment added to the Bugzilla thread. "Not sure what to do about that." The same hacker who posted exploit code last week was one of two who claimed to have created the newest attack code.

Gal declined to suggest solutions about hacker trawling. "I am not comfortable talking about what goes wrong and why since that might reveal other, potentially even more severe problems in some cases," he said.

There is as yet no fix for the crash bug now being investigated.

According to Mozilla's Web site, it's planning to release another update to Firefox -- tagged as 3.5.2 -- by the end of this month or in early August.